There is an emerging market for dispersed nanoparticles in various fields of the opto-electronic industry. Some typical applications of for example metal selenide nanocrystals are solid state lighting, printed photovoltaics, photodetectors, biolabeling etc.
Over the last 20 year, various solution-based routes for the formation of metal selenide nanocrystals have been developed. One technique that can be used is a heating up technique, wherein the components are initially mixed and wherein thereafter heating of the mixture is performed. An example thereof is described in “Synthesis of CdSe and CdTe Nanocrystals without precursor injection” by Yang et al. in Angew. Chem. 2005, volume 117, p 6870-6873. Another technique that is often used is the hot injection method. The hot injection method in which metal and selenium precursors are injected in a hot solvent to form metal selenide nanoparticles showed to offer most control over size and shape. It has resulted in synthesis recipes for nanocrystals of, e.g., CdSe and PbSe with size dispersions as low as 5% and accessible size ranges from 2 to more than 20 nm. With respect to upscaling, the hot injection synthesis faces a number of issues.
Most current approaches use precursors that are oxygen or water sensitive, such as trioctylphosphine selenium or bistrimethylsilylselenide. The latter implies that, for the current synthesis methods, there is a need for a controlled inert atmosphere, such as for example a nitrogen or argon atmosphere.
Another problem that typically occurs when using hot injection is that the reactions are often very slow to run to completion. Attempts to adjust the yield of a hot injection synthesis have been limited. In general, attaining a high synthesis yield in a reasonable time span requires highly reactive precursors or reaction intermediates. The yield therefore often is a property of the reaction. For instance, with trioctylphosphine selenium and cadmium carboxylates, full yield is reached with minutes to hours. Homogeneously dissolved selenium on the other hand is a precursor with a low reactivity, resulting in a low reaction yield in combination with cadmium carboxylates. A solution introduced in the case of the PbSe synthesis, is the use of additives that speed up the reaction.
Still another problem that occurs when using hot injection methods is that these often require expensive chemicals, such as trioctylphospine. Jasieniak et al. describe in “Phosphine-Free Synthesis of CdSe Nanocrystals” a synthesis method that is phosphine free, but this results in a slow synthesis and the need for an inert nitrogen atmosphere.
There is still a need for an efficient synthesis method for metal selenides.